Propeller shaft support



Nov. 22, 1966 J. PLUM 3,286,681

PROPELLER SHAFT SUPPORT Filed Nov. 30, 1964 Q3 2 L w INVENTOR.

JOHN PLUM ATTY.

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AGENT United States Patent O 3,286,681 PROPELLER SHAFT SUPPORT JohnPlum, Montgomery County, Md. (5402 Tuscarawas Road, Washington, D.C.20016) Filed Nov. 30, 1964, Ser. No. 414,927 5 Claims. (Cl. 115-34) Theinvention described herein may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

The present invention relates to improvements in the installation ofpropeller shafts. One object of the invention is to increase theefficiency of the propeller. Another object is to reduce the vibrationalforces which produce noise and fatigue failures in the propulsionsystem.

It should be realized that as a craft moves thru water, the shaft aswell as the hull will be greatly affected by dynamic forces. This isespecially true in the case where the propeller shaft is inclined towardthe flow under the bottom of a high-speed craft. If the shaft isinadequately supported, the dynamic lift on the shaft will bend it upinto a curve which is concave downwards. This will result in an angle ofthe propeller axis which is greater than the initial value of the shaftangle.

The detrimental effects of having the propeller axis at an angle to theinflow arise from the fact that this causes an increase in the angle ofattack of the blades moving downward and causes a decrease in the angleof attack of the blades moving upward, which in turn will move thecenter of the thrust out of line with the center of the propeller.Furthermore, since the propeller has several blades, when it is rotatingrapidly thousands of vibrations per minute will be transmitted to thehull through the propeller bracket, and there will be thousands ofthrust-impulses tending to twist or bend the shaft in the vicinity ofthe propeller bracket. The many examples of fatigue failure of thesebrackets are, therefore, readily understandable.

Increasing the angle of attack of the blades moving down and decreasingthe angle of attack of the blades moving up will not only reduce theeffective area of the propeller but it may overload one side and causecavitation.

It has been explained that a concave bend in an inclined shaft willaggravate all the detrimental effects which are produced by the intialangular infiOW. A convex bend will therefore have the opposite effect.In fact, a correct amount of convex curvature of an inclined shaft will:

(a) Distribute the thrust evenly between the two sides of the propeller,thereby increasing its effective area.

(b) Increase the cavitation-inception speed of the propeller.

(c) Eliminate the principal factor producing vibrationa1 forces andnoise.

(d) Remove the obstruction of the inclined shaft from the inflow to thepropeller.

The advantages of a convex curvature in an inclined shaft have beendiscovered accidently by many pilot-s of single-screw speedboats. Thisaccidental discovery occurs when the propeller bracket fails at highspeed. The failure of this strut will immediately permit the shaft tobend upward in the convex curvature of a cantilever, which will resultin a sudden speed increase and a cessation of all vibration and noise.

The advantages resulting from a broken propeller bracket and theassociated convexly bent propeller shaft have been recognized at page127 of the 1961 volume of the Transaction of the Royal Institute ofNaval Architects. It has been determined by the inventor that thedynamic lift on an inclined shaft which is convexly bent "ice (by, forinstance, a failure of the propeller bracket) will enable the shaft toWith-stand an additional buckling force which is four times themagnitude of the lift force, whereas, the dynamic lift on a concavelybent shaft will have the opposite effect. This explains the phenomenonthat the shaft does not buckle when the propeller bracket breaks.However, the unsupported shaft will eventually undergo fatigue failuredue to stresses on the shaft inside the hull caused by reverse bending.

It can therefore be concluded that in order to prevent buckling from ahigh thrust-load, a convexly bent shaft needs only a minimum number ofpoints of support, Whereas a shaft installed straight requires a greatmany closely space-d struts in order to prevent concave bending at highspeed.

It has been explained that a broken propeller bracket will produce thecurvature of a single cantilever (similar to that shown in Figure 5,page 96 of Popular Boating of March 1962) whereas in the presentinvention a more uniform bending is obtained by installing the shaftwith the curvature of a double cantilever. The shaft is initiallysupported at tWo points, i.e., at the propeller and at a suitable pointforwardly in the craft. Then the shaft is prestressed downward by anintermediate strut located approximately midway between the two initialpoints of support. In this manner, two cantilevers are started in themiddle region of the shaft, one of which will taper off toward thepropeller while the other will taper ofi forwardly. The thrust of thepropeller tends to hold the shaft in its convex bend, thus reducingstresses at the point of the intermediate strut. According to theinvention bending stresses are reduced in the shaft by about fiftypercent. Since fatigue failures are caused by alternating tensionforces, the compression force produced by the thrust on the shaft-columndoes not contribute to fatigue. Ignoring the effect of torque, theresult is that fatigue stresses will not affect the life span of a shaftof reasonable length.

Other advantages as well as objects and features of the presentinvention will be better understood by referring to the followingdrawings and accompanying specification in which like numerals representcorresponding parts and in which:

FIG. 1 is a simplified side view of a prior art Watercraft depictingpropeller shaft disposition;

FIG. 2 is a simplified side view depicting propeller shaft behaviorunder certain conditions; and

FIG. 3 is a simplified side view of a propeller shaft arrangementaccording to the present invention.

It is to be understood that the principles of the present inventionapply to surface-operating water craft of any configuration, e.g.hydrofoils, planing hulls and displacement hulls.

The prior art hydrofoil configuration shown in FIG. I typically employsa straight inclined propeller shaft 11 carrying a propeller 12 andsupported at its forward end either directly at a power plant 13 or by ashaft log 15, at an intermediate point by a strut 17, and at its aft endby a propeller bracket 19. When the craft of FIG. 1 moves in the waterat high speed, the shaft 11 is bent under hydrodynamic forces and underthe influence of the intermediate strut into an S shaped curve(indicated by broken lines 21). This S curve has a concave portion 23and a convex portion 25. Thus, the propeller, located at or near the endof the shaft is caused to have a greater angle of incidence to flow(indicated in broken lines) reducing propeller efficiency and increasingvibration and noise. If there were no intermediate strut, the entireshaft would tend to be concave in shape, producing the samedisadvantageous propeller orientation. Stresses similar to thosedescribed above would be produced in a propeller shaft mounted on othertypes of hulls, such as planing and displacement hulls.

As shown in FIG. 2, if both the strut 17 and bracket 19 failed or weredeliberately removed, at high speeds the shaft would act as a singlecantilever 27 and would be lifted by the propeller to the raisedposition shown. In FIG. 2 most of the shaft curvature is seen to occurin the region 29 of the propeller support which may be located either atthe take-off 13 or at the shaft log 15. Thus, even with a flexible shaftin the arrangement of FIG. 2 there could be early failure of the shaftdue to extreme stresses inside the hull at the region 29 of largecurvature.

According to the present invention, a version of which is shown arrangedon the generally indicated watercraft 37 shown in FIG. 3, there isprovided a prestressed bent propeller shaft 41 having essentiallyuniform curvature. The craft 37 may be a planing craft, displacementhull, or a hydrofoil having foils 39, 40 as indicated. This uniformcurvature is produced by a depending intermediate strut 43 of sufficientlength to produce a doubly cantilevered bend along the central portionof the shaft. Of course, a bearing 45 of any suitable design is mountedat the tip of the strut 43 to carry the shaft. For example, alongitudinally grooved rubber bearing of the type manufactured by theGoodrich Rubber Co. may be used.

The extent of curvature imposed on the shaft depends upon the length andmaerial characteristics of the shaft. For purposes of example, and notby way of limitation, it has been found that for a boat approximately 30feet in length having an overall shaft length of about 12 to 18 feet,the curvature may be formed by displacing the portion of the shaft about3 to 8 inches below a straight line joining the forward and aft centralshaft support points. A shaft of Monel metal of 1% inches diameter maybe employed.

It is to be understood that the propeller shaft support arrangementaccording to the present invention may be employed on watercraft ofvarious sizes and configurations, for example, on multi-hulled craft andon larger ships.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. For a watercraft, propeller shaft support means comprising:

downwardly extending propeller shaft bracket means located at an aftportion of the craft;

power take-off means comprising at least one fiexible propeller shafthaving a propeller at its aft end and having at its forward end a firstload bearing point located at least substantially forwardly of saidbracket and at its aft end a second load point at the bracket; means forsupporting said at least one shaft in a 5 prestressed condition in aconvex downward curve between said first and second load points, saidmeans comprising a further strut extending from the craft to a positionof said shaft intermediate said first and second load points toestablish an intermediate load point for said shaft, said further struthaving bearing means for carrying said shaft;

whereby the inflow of water to the propeller is essentially parallel tothe shaft at the propeller location.

2. The watercraft according to claim 1 wherein the propeller is mountedforwardly of said bracket.

3. For a watercraft having a power plant mounted Within the watercraftsubstantially forward of the stern, said watercraft further having arudder bracket located at the stern and a bearing in the bracket forsupporting a propeller shaft coupled at its forward end to the powerplant, a propeller shaft arrangement comprising:

a flexible propeller shaft inclined downwardly toward the bracket andsupported at the bracket and power plant;

strut means depending from said hull to a position of the inclinedpropeller shaft intermediate the rudder bracket and the power plant;

bearing means mounted on said strut means for carrying said shaft;

said strut means being of such dimension as to displace the shaftdownwardly to form a continuous convex downward curve between thebracket and power plant;

whereby the inflow of water to the propeller is essentially parallel tothe shaft at the propeller location.

4. The propeller shaft arrangement according to claim 3 wherein saidstrut means comprises a single rigid strut.

5. The propeller shaft arrangement according to claim 40 3 wherein saidwatercraft comprises a hull.

References Cited by the Examiner UNITED STATES PATENTS FERGUS S.MIDDLETON, Primary Examiner. MILTON BUCHLER, Examiner.

T. MAJOR, Assistant Examiner.

1. FOR A WATERCRAFT, PROPELLER SHAFT SUPPORT MEANS COMPRISING:DOWNWARDLY EXTENDING PROPELLER SHAFT BRACKET MEANS LOCATED AT AN AFTPORTION OF THE CRAFT; POWER TAKE-OFF MEANS COMPRISING AT LEAST ONEFLEXIBLE PROPELLER SHAFT HAVING A PROPELLER AT ITS AFT END AND HAVING ATITS FORWARD END A FIRST LOAD BEARING POINT LOCATED AT LEASTSUBSTANTIALLY FORWARDLY OF SAID BRACKET AND AT ITS AFT END A SECOND LOADPOINT AT THE BRACKET; MEANS FOR SUPPORTING SAID AT LEAST ONE SHAFT IN APRESTRESSED CONDITION IN A CONVEX DOWNWARD CURVE BETWEEN SAID FIRST ANDSECOND LOAD POINTS, SAID MEANS COMPRISING A FURTHER STRUT EXTENDING FROMTHE CRAFT TO A POSITION OF SAID SHAFT INTERMEDIATE SAID FIRST AND SECONDLOAD POINTS TO ESTABLISH AN INTERMEDIATE LOAD POINT FOR SAID SHAFT, SAIDFURTHER STRUT HAVING BEARING MEANS FOR CARRYING SAID SHAFT; WHEREBY THEINFLOW OF WATER TO THE PROPELLER IS ESSENTIALLY PARALLEL TO THE SHAFT ATTHE PROPELLER LOCATION.